Atmospheric carbon will then start to increase more rapidly, accelerating climate change, the researchers warn.

Vegetation acts as a ''carbon sink'', a natural system that takes carbon dioxide from the air and traps it. The Amazon rainforest and the vast belt of coniferous boreal forest that rings the northern hemisphere both act as powerful carbon sinks.

Initially, higher levels of carbon dioxide in the atmosphere will stimulate more plant growth. But the impact of a warmer world will gradually counterbalance this trend until saturation point is reached, say the scientists.

Global warming of 4 degrees will result in the Earth's land vegetation becoming dominated by negative effects, such as ''moisture stress'', caused by plant cells getting too little water.

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At this point, further absorption of atmospheric carbon is likely to flatline, according to the research published in the journal Proceedings of the National Academy of Sciences.

Lead scientist Andrew Friend, of Cambridge University, said: ''The impacts of climate on vegetation will affect biodiversity and ecosystem status around the world.''

The study used seven different computer models to simulate the effects of global warming on plant life.